1. Critical fields, vortex melting and the irreversibility line in quasi 2D organic superconductors Braunen E. Smith K. Cho, I. Mihut and C. C. Agosta Department of Physics, Clark University S. W. Tozer, T. P. Murphy and E. C. Palm The National High Magnetic Field Laboratory, Tallahassee J. A. Schlueter Materials Science Division, Argonne National Laboratory We acknowledge support from DOE grant #ER46214.

2. Introduction –Organic Superconductor  -(ET) 2 Cu(NCS) 2 –Experimental Technique Vortex Melting Point –Smoothing of Melting Point in Parallel Orientation Irreversibility Line –Comparison to Data from M.M. Mola, S. Hill, J.S. Brooks, and J.S. Qualls, Phys. Rev. Lett. 86, 2130 (2001). Upper Critical Field –DC versus Pulsed Field Conclusions

3.  -(ET) 2 Cu(NCS) 2 Layered type-II superconductor with BEDT- TTF molecules packed in planes separated by the smaller poorly conducting Cu(NCS) 2 anion layers. This structure is similar to HTS (high temperature superconductors), but samples are intrinsically cleaner. Lower T c (~10 K) and H c2 than HTS which allows for more of the temperature/field space to be examined. M.M. Mola, S. Hill, J.S. Brooks, and J.S. Qualls, Phys. Rev. Lett. 86, 2130 (2001).

4. Experimental Technique The National High Magnetic Field Laboratory, Tallahassee – Portable Dilution Refrigerator – 33 T DC Magnet TDO Technique to probe change in rf penetration depth as magnet field is changed – The change in frequency is proportional to the change in penetration depth – Resonant frequency ~200MHz

5. Vortex Melting Point As the sample orientation approaches parallel (90°) the melting transition becomes less obvious and changes field.

6. Irreversibility Line Tinkham Eq. (2D Ginzburg-Landau): Solid Line: B irr  =3.6 T B irr|| =32 T Dashed Line: B irr  =3.6 T B irr|| =26 T M.M. Mola, S. Hill, J.S. Brooks, and J.S. Qualls, Phys. Rev. Lett. 86, 2130 (2001).

7. Irreversibility Line M.M. Mola, S. Hill, J.S. Brooks, and J.S. Qualls, Phys. Rev. Lett. 86, 2130 (2001). > 0.5° angle resolution

8. Upper Critical Field (B c2 ) All pulsed field data taken at Clark University Pulsed Field Lab Originally thought to be related to time constant of vortices. Mystery: there should only be small vortex effects in parallel orientation. Difference in B c2 not seen in CeCoIn 5

9. Conclusions TDO technique can be used to examine vortex melting –As the sample nears the parallel orientation vortex melting transition smoothes out. Irreversibility point for parallel sample orientation found to be 6T lower than previously predicted. –This can be explained with a slight correction to the angle reported by Mola and Hill. Difference between pulsed field and DC field data possibly due to time constant of moving vortices. The difference in critical field for parallel orientation is a mystery. The melting transition is smoothed, the hysteresis in the irreversibility line is very small and the flux jumps are no longer present, i.e., all the characteristics of the vortex dynamics have disappeared. Why is the difference in critical field still present?